Some of the events that take place during the differentiation of human and mouse erythroleukemic cells include an initiation of the synthesis of two membrane proteins; spectrin and glycophorin, a decrease in lectin-mediated agglutinability, changes in surface morphology and an asymmetric division into a nucleus-containing cell and a vesicle containing hemoglobin and erythrocyte membrane proteins. We shall investigate the mechanism of these changes. The initiation and subsequent cessation of membrane protein synthesis will be examined by isolation of the messanger RNA (mRNA) coding for these proteins, and its translation in vitro. We shall be looking for changes in the mRNA levels during differentiation. During translation it is thought that many membrane glycoproteins are synthesized as precursor molecules with molecular weights greater than the final product. Using our in vitro protein synthesis system and specific antisera, we shall investigate whether this is the case for erythrocyte membrane glycoproteins. The mechanism of the asymmetric division of erythroleukemic cells will be investigated by electron and fluorescence microscopy of differentiating cells. We shall be looking for cytoskeletal structures that may be involved in the production of the vesicle. The effects of various drugs on the vesicle will be examined in an attempt to elucidate the mechanism of the sequestration of erythrocyte membrane proteins into this structure.